text
stringlengths 62
2.94k
|
|---|
Effective Test Generation Using Pretrained Large Language Models and Mutation Testing ; One of the critical phases in software development is software testing. Testing helps with identifying potential bugs and reducing maintenance costs. The goal of automated test generation tools is to ease the development of tests by suggesting efficient bugrevealing tests. Recently, researchers have leveraged Large Language Models LLMs of code to generate unit tests. While the code coverage of generated tests was usually assessed, the literature has acknowledged that the coverage is weakly correlated with the efficiency of tests in bug detection. To improve over this limitation, in this paper, we introduce MuTAP for improving the effectiveness of test cases generated by LLMs in terms of revealing bugs by leveraging mutation testing. Our goal is achieved by augmenting prompts with surviving mutants, as those mutants highlight the limitations of test cases in detecting bugs. MuTAP is capable of generating effective test cases in the absence of natural language descriptions of the Program Under Test PUTs. We employ different LLMs within MuTAP and evaluate their performance on different benchmarks. Our results show that our proposed method is able to detect up to 28 more faulty humanwritten code snippets. Among these, 17 remained undetected by both the current stateoftheart fully automated test generation tool i.e., Pynguin and zeroshotfewshot learning approaches on LLMs. Furthermore, MuTAP achieves a Mutation Score MS of 93.57 on synthetic buggy code, outperforming all other approaches in our evaluation. Our findings suggest that although LLMs can serve as a useful tool to generate test cases, they require specific postprocessing steps to enhance the effectiveness of the generated test cases which may suffer from syntactic or functional errors and may be ineffective in detecting certain types of bugs and testing corner cases PUTs. |
Mass Extinction in a Simple Mathematical Biological Model ; Introducing the effect of extinction into the socalled replicator equations in mathematical biology, we construct a general model of ecosystems. The present model shows mass extinction by its own extinction dynamics when the system initially has a large number of species diversity. The extinction dynamics shows several significant features such as a power law in basin size distribution, induction time, etc. The present theory can be a mathematical foundation of the speciesarea effect in the paleontologic theory for mass extinction. |
Beat Frequency Modulation of T Tauri Accretion Rates ; A general model of magnetically controlled accretion onto T Tauri stars is presented. In this model the magnetic field is oriented arbitrarily in relation to the star's rotation axis. The resultant interplay between the magnetic field and accretion disc causes a variable accretion rate. The dominant timescale of this variability is the beat frequency between the stellar rotation frequency and the orbital frequency at the magnetosphere boundary. This model is analogous to that developed to explain quasiperiodic oscillations in lowmass Xray binaries. |
T Tauri variability in the context of the beatfrequency model ; We examine the implications of a beat frequency modulated model of T Tauri accretion. In particular we show that measurements of the variability of accretion generated lines can be used in conjunction with existing photometry to obtain a measurement of the underlying photospheric and disc flux. This provides an independent way of checking spectral energy distribution modelling. In addition, we show how spectroscopy of T Tauri stars can reveal the inclination angle between the magnetic axis and the plane of the disc. |
Inflationary cosmology and structure formation ; These lectures cover the basics of inflationary models for the early universe, concentrating particularly on the generation of density fluctuations from scalarfield dynamics. The subsequent gravitational dynamics of these fluctuations in dark matter in a Friedmann model are described, leading to a review of the current situation in confronting inflationary models with the latest data on the clustering of galaxies and other measures of largescale structure. |
Topological Defect Models of UltraHigh Energy Cosmic Rays ; We give an overview over models in which cosmic rays above 1 EeV 1018 eV are produced by the decay of supermassive X particles released from topological defects possibly created in cosmological phase transitions. We note that, for an interesting particle physics parameter range, these models are still consistent with current data, and discuss signatures for the topological defect mechanism which can be tested by the next generation experiments. |
Periodic Orbits in Triaxial Galaxies with Weak Cusps ; The orbital structure of triaxial models with weak central density cusps, rhopropto rgamma, gamma 1, is investigated. The stability of the x long axis orbit and hence the existence of box orbits depends sensitively on gamma; the range of model shapes for which the xaxis orbit is stable becomes progressively smaller as gamma approaches one. The banana and fish boxlets in the xz long axisshort axis plane are stable over a wide range of model parameters. The boxlets in the xy and yz planes are generally vertically unstable. |
Issues for the Next Generation of Galaxy Surveys ; I argue that the weight of the available evidence favours the conclusions that galaxies are unbiased tracers of mass, the mean mass density excluding a cosmological constant or its equivalent is less than the critical Einsteinde Sitter value, and an isocurvature model for structure formation offers a viable and arguably attractive model for the early assembly of galaxies. If valid these conclusions complicate our work of adding structure formation to the standard model for cosmology, but it seems sensible to pay attention to evidence. |
A Model for the Density Distribution of Virialized CDM Halos ; An analytic collapse model for the formation and density distribution of virialized cold dark matter halos is proposed. Hierarchical structure formation is taken into account explicitly. Monte Carlo methods are used to generate samples of mass histories of virialized halos. The mean density distribution found from the collapse model is in good agreement with numerical results in the mass range from 1011Modot to 1015Modot and in the radial range form 0.05 r200 to r200. |
Deductivereductive determination of the model of our observed Universe ; According to the observations, in our expansive and isotropic relativistic Universe for the gravitational phenomena in a Newtonian approximation the Newtonian nonmodified relations are valid. The Friedmann general equations of isotropic and homogeneous universe dynamics describe an infinite number of models of expansive and isotropic relativistic universe in the Newtonian approximation, but only in one of them the Newtonian nonmodified relations are valid. These facts give till now not considered possibility for unambiguous deductivereductive determination of the Friedmannian model, which describes our observed Universe. |
A generalized inflation model with cosmic gravitational waves ; We propose a Lambdainflation model which explains a large fraction of the COBE signal by cosmic gravitational waves. The primordial density perturbations fulfil both the contraints of largescale microwave background and galaxy cluster normalization. The model is tested against the galaxy cluster power spectrum and the highmultipole angular CMB anisotropy. |
A Counterexample to Claimed COBE Constraints on Compact Toroidal Universe Models ; It has been suggested that if the Universe satisfies a flat, multiply connected, perturbed FriedmannLemaitre model, then cosmic microwave background data from the COBE satellite implies that the minimum size of the injectivity diameter shortest closed spatial geodesic must be larger than about two fifths of the horizon diameter. To show that this claim is misleading, a simple T2 times R universe model of injectivity diameter a quarter of this size, i.e. a tenth of the horizon diameter, is shown to be consistent with COBE four year observational maps of the cosmic microwave background. This is done using the identified circles principle. |
Theoretical Models of Photodissociation Fronts ; Observations of H2 line emission have revealed higherthanexpected gas temperatures in a number of photodissociation fronts. We discuss the heating and cooling processes in photodissociation regions. Observations of NGC 2023 are compared to a theoretical model in which there is substantial gas at temperatures T 5001000K heated by photoelectric emission and collisional deexcitation of H2. In general the model successfully reproduces the observed H2 line emission from a wide range of energy levels. The observed SiII34.8um emission appears to indicate substantial depletion of Si in NGC 2023. |
Cosmological Parameter Estimation from CMB Experiments ; I review the general aspects of cosmological parameter estimation from observations of the cosmic microwave background CMB temperature anisotropies in the framework of inflationary adiabatic models. The most recent CMB datasets are starting to give good constraints on the relevant parameters of inflationary adiabatic models. They point toward a model consistent with the basic predictions of inflation a nearly flat universe, with a nearly scale invariant spectrum of primordial fluctuation. |
Two Types of Radio Galaxies A New Approach ; We do not fully understand the dynamics and evolution of a radio galaxy. Models of classical double Type II sources are in a reasonable state, but these objects are rare. Nontype II sources generically called Type I are far more common, but much less well understood. In this paper I use the data to suggest possible new models for Type I sources, and discuss the physical questions which these new models raise. |
Scalar field models for an accelerating universe ; I describe a new class of quintessenceCDM models in which late time scalar field oscillations can give rise to both quintessence and cold dark matter. Additionally, a versatile ansatz for the luminosity distance is used to reconstruct the quintessence equation of state in a model independent manner from observations of high redshift supernovae. |
Chemical Evolution and Starbursts ; The first part of this paper deals with the impact of nonsolar and for latetype, dwarf, and high redshift galaxies generally subsolar abundances on the interpretation of observational data for starburst galaxies. It points out the differences in colors, luminosities, emission lines, etc. obtained from a model using low metallicity input physics for a starburst on top of the stellar population of a galaxy as compared to an otherwise identical model using solar metallicity input physics only. The second part deals with the chemical evolution during a starburst and contrasts model predictions with observational clues. |
The Role of Active Regions in the Generation of Torsional Oscillations ; We present a model for torsional oscillations where the inhibiting effect of active region magnetic fields on turbulence locally reduces turbulent viscous torques, leading to a cycle and latitudedependent modulation of the differential rotation. The observed depth dependence of torsional oscillations as well as their phase relationship with the sunspot butterfly diagram are reproduced quite naturally in this model. The resulting oscillation amplitudes are significantly smaller than observed, though they depend rather sensitively on model details. Meridional circulation is found to have only a weak effect on the oscillation pattern. |
A Model of Varying Fine Structure Constant and Varying Speed of Light ; The recent evidence for a cosmological evolution of the fine structure constant alphae2hbar c found from an analysis of absorption systems in the spectra of distant quasars, is modelled by a cosmological scenario in which it is assumed that only the speed of light varies. The model fits the spectral line data and can also lead to a solution of the initial value problems in cosmology. |
Cosmic acceleration from effective forces ; Accelerated expansion of the Universe may result from an antifrictional force that is selfconsistently exerted on cold dark matter CDM. Cosmic antifriction is shown to give rise to an effective negative pressure of the cosmic medium. While other models introduce a component of dark energy besides standard'' CDM, we resort to a phenomenological onecomponent model of CDM with internal selfinteractions. We demonstrate how the dynamics of the LambdaCDM model may be recovered as a special case of cosmic antifriction. |
Potentials and distribution functions to be used for dynamical modeling with GAIAlike data ; We present new tools to establish axisymmetric equilibrium models of the Milky Way. The models we wish to establish are pairs V,F where V is the gravitational potential generated by the whole mass distribution including the dark matter, and F is the distribution function in phase space for latetype tracer stars. We present a set of Stackel potentials that fit some fundamental parameters of the Milky Way mass density in the solar neighbourhood and Oort constants. Then we define new component distribution functions that can be combined with these potentials in order to reproduce kinematical data like those that will be provided by GAIA. |
Supernovae, CMB, and Gravitational Leakage into Extra Dimensions ; We discuss observational constraints coming from CMB and type Ia supernovae, for the model of accelerated universe produced by gravitational leakage into extra dimensions. Our fits indicate that the model is currently in agreement with the data. We also give the equations governing the evolution of cosmological perturbations. Future observations will be able to severely constrain the model. |
MHD Models of Planetary Nebulae Review ; Hydrodynamical HD simulations played an important role in understanding the dynamics and shaping of Planetary Nebulae PNe in the past century. However, HD solutions are just a first order approach. The new millennium arrives with the generalized understanding that the effects of magnetic fields i.e., MHD are necessary to study the dynamics of PNe. Thus, Bfields introduce a whole new number of physical possibilities for the modeling. We here review recent advances in MHD modeling of PNe. |
Loop Corrections to Scalar Quintessence Potentials ; The stability of scalar quintessence potentials under quantum fluctuations is investigated for both uncoupled models and models with a coupling to fermions. We find that uncoupled models are usually stable in the late universe. However, the coupling to fermions is severely restricted. We check whether a graviton induced fermionquintessence coupling is compatible with this restriction. |
Asymptotic behavior of a stratified perturbation in a three dimensional expanding Universe ; The nonlinear evolution of a stratified perturbation in a three dimensional expanding Universe is considered. A general Lagrangian scheme Q model is introduced and numerical investigations are performed. The asymptotic contraction of the core of the agglomeration is studied. A powerlaw scaling is detected and an heuristic interpretation of the numerical findings is provided. An asymptotic equation for the multistream velocity flow is derived and it is shown to agree quantitatively with the dynamics of the Q model. The relation to the adhesion model is discussed. |
Temperature Profiles and Spectra of Accretion Disks around Rapidly Rotating Neutron Stars ; We calculate temperature profiles and Xray spectra of accretion disks around rapidly rotating neutron stars considering the full effect of general relativity. Computed disk temperatures and luminosities are compared with the EXOSAT data to constrain the properties of five lowmassXraybinary sources. We fit our modelspectra with an analytical function, which can in turn be used for routine spectral fitting work. Our equationofstate dependent spectral model may be useful to constrain the equationofstate models of neutron stars. We also compare the properties of a rotating neutron star with those of a rotating strange star with the hope of giving a possible way to identify a strange star, which will be important for the verification of strangequarkmatter hypothesis. |
Antimatter in the Universe ; Different scenarios of baryogenesis are briefly reviewed from the point of view of possibility of generation of cosmologically interesting amount of antimatter. It is argued that creation of antimatter is possible and natural in many models. In some models not only antihelium may be produced but also a heavier antielements and future observations of the latter would be critical for discovery or establishing stronger upper limits on existence of antimatter. Incidentally a recent observation of ironrich quasar may present a support to one special model of antimatter creation. |
Galactic Evolution along the Hubble Sequence ; A generalization of the multiphase chemical evolution model applied to a wide set of theoretical galaxies is shown. This set of models has been computed by using the socalled Universal Rotation Curve from Persic, Salucci Steel to calculate the radial mass distributions of each theoretical galaxy. By assuming that the molecular cloud and star formation efficiencies depend on the morphological type of each galaxy, we construct a biparametric grid of models whose results are valid in principle for any spiral galaxy, of given maximum rotation velocity or total mass, and morphological type. |
Equilibrium Models of Galaxy Clusters with Cooling, Heating and Conduction ; It is generally argued that most clusters of galaxies host cooling flows in which radiative cooling in the centre causes a slow inflow. However, recent observations by Chandra and XMM conflict with the predicted cooling flow rates. Amongst other mechanisms, heating by a central active galactic nucleus and thermal conduction have been invoked in order to account for the small mass deposition rates. Here, we present a family of hydrostatic models for the intracluster medium where radiative losses are exactly balanced by thermal conduction and heating by a central source. We describe the features of this simple model and fit its parameters to the density and temperature profiles of Hydra A. |
The DiskJet Connection in Microquasars and AGN ; We propose a new model for the diskjet connection in black hole xray transients and active galactic nuclei. In our model, the inner part of the accretion disk around the central black hole switches between two states. In one state, the accretion energy is dissipated locally some within the disk, some within a corona to produce the observed disk luminosity. In the second state, the accretion energy is deposited into the bulk flow of a relativistic jet. We associate the transition between the two states with the generation of a global, poloidal magnetic field. We show that this model can explain the observed behavior of black hole accretors. |
Another possible interpretation of SN 1a data without kinematics Comments on the paper astroph0402512 by A. Riess et al ; It is shown here that for redshifts z 0.5 the luminosity distance, which is predicted in author's model astroph0005084 v2, fits well supernova observational data of astroph0402512 by A.Riess et al. Discrepancies for higher z would be explained in the model as a result of specific deformation of SN spectra due to a discrete character of photon energy losses. The model does not require any dark energy; it is based on the conjecture that gravitons are superstrong interacting particles fulfilling a flat nonexpanding universe. |
Dark Energy in Chains ; Dark energy affects the CMB through its perturbations and affects both CMB and SnIa through its background evolution. Using recent CMB and SnIa data sets, together with the most general parameterization of the dark energy equation of state available, we find that today w 0.8 2 sigma. We also find that the value of the normalization of the power spectrum on cluster scales, sigma8, can be used to discriminate between dynamical models of dark energy Quintessence models and a cosmological constant model Lambda CDM. |
Largescale inhomogeneities in modified Chaplygin gas cosmologies ; We extend the homogeneous modified Chaplygin cosmologies to largescale perturbations by formulating a Zeldovichlike approximation. We show that the model interpolates between an epoch with a soft equation of state and a de Sitter phase, and that in the intermediate regime its matter content is simply the sum of dust and a cosmological constant. We then study how the largescale inhomogeneities evolve and compare the results with cold dark matter CDM, LambdaCDM and generalized Chaplygin scenarios. Interestingly, we find that unlike the latter, our models do always resemble LambdaCDM. |
QSSC reexamined for the newly discovered SNe Ia ; We examine the possible consistency of the quasisteady state model with the newly discovered SNe Ia. The model assumes the existence of metallic dust ejected from the SNe explosions, which extinguishes light travelling over long distances. We find that the model shows a reasonable fit to the data, which improves if one takes account of the weak gravitational lensing effect of the SNe which have been observed on the brighter side. |
Acceleration of the universe with a simple trigonometric potential ; In this paper we investigate the quintessence model with a minimally coupled scalar field in the context of recent supernovae observations. By choosing a particular form of the deceleration parameter q, which gives an early deceleration and late time acceleration for dust dominated model, we show that this sign flip in q can be obtained by a simple trigonometric patential. The early matter dominated model expands with q12 as desired and enters a negative q phase quite late during the evolution. |
Radial Velocity Jitter in Stars from the California and Carnegie Planet Search at Keck Observatory ; I present an empirical model for predicting a star's radial velocity jitter from its BV color, activity level, and absolute magnitude. This model is based on observations of 450 well observed stars from Keck Observatory for the California and Carnegie Planet Search Program. The model includes noise from both astrophysical sources and systematic errors, and describes jitter as generally increasing with a star's activity and height above the main sequence. |
Is the island universe model consistent with observations ; We study the island universe model, in which initially the universe is in a cosmological constant sea, then the local quantum fluctuations violating the null energy condition create the islands of matter, some of which might corresponds to our observable universe. We examine the possibility that the island universe model is regarded as an alternative scenario of the origin of observable universe. |
Primordial black hole constraints on nongaussian inflation models ; We determine the abundance of primordial black holes PBHs formed in the context of nongaussian models with primordial density perturbations. We consider models with a renormalized chi2 probability distribution function parametrized by the number, nu, of degrees of freedom. We show that if nu is not too large then the PBH abundance will be altered by several orders of magnitude with respect to the standard gaussian result obtained in the nu to infty limit. We also study the dependence of the spectral index constraints on the nature of the cosmological perturbations for a powerlaw primordial power spectrum. |
NonGaussianity from Broken Symmetries ; Recently we studied inflation models in which the inflaton potential is characterized by an underlying approximate global symmetry. In the first work we pointed out that in such a model curvature perturbations are generated after the end of the slowroll phase of inflation. In this work we develop further the observational implications of the model and compute the degree of nonGaussianity predicted in the scenario. We find that the corresponding nonlinearity parameter, fNL, can be as large as 102. |
A model for atomic and molecular interstellar gas The Meudon PDR code ; We present the revised Meudon'' model of Photon Dominated Region PDR code, presently available on the web under the Gnu Public Licence at httparistote.obspm.frMIS. General organisation of the code is described down to a level that should allow most observers to use it as an interpretation tool with minimal help from our part. Two grids of models, one for low excitation diffuse clouds and one for dense highly illuminated clouds, are discussed, and some new results on PDR modelisation highlighted. |
Is space expanding in the Friedmann universe models ; The interpretation of the expanding universe as an expansion of space has recently been challenged. From the geodesic equation in Friedmann universe models and the empty Milne model, we argue that a Newtonian or special relativistic analysis is not applicable on large scales, and the general relativistic interpretation in terms of expanding space has the advantage of being globally consistent. We also show that the cosmic redshift, interpreted as an expansion effect, containts both the Doppler effect and the gravitational frequency shift. |
Mechanical Model for Relativistic Blast Waves ; Relativistic blast waves can be described by a mechanical model. In this model, the blast the compressed gas between the forward and reverse shocks is viewed as one hot body. Equations governing its dynamics are derived from conservation of mass, energy, and momentum. Simple analytical solutions are obtained in the two limiting cases of ultrarelativistic and nonrelativistic reverse shock. Equations are derived for the general explosion problem. |
Just dust About the inapplicability of rotating dust solutions as realistic galaxy models ; Solutions of the stationary axisymmetric Einstein equations describing the interior of circularly rotating dust are investigated in order to study their potential applicability as galaxy models. It is shown that such interior solutions cannot be used as global metrics without becoming unphysical in certain regions of space. Although definite results concerning the nonexistence of a continuation into a vacuum can only be derived for interior solutions describing rigidly rotating dust, the present analysis exhibits that the case of nonrigidly rotating dust would in general also be inadequate as a physically reasonable galaxy model. |
The Masses of LateType WN Stars ; We present recent results for galactic WNL stars, obtained with the new Potsdam WolfRayet PoWR hydrodynamic model atmospheres. Based on a combination of stellar wind modeling and spectral analysis we identify the galactic WNL subtypes as a group of extremely luminous stars close to the Eddington limit. Their luminosities imply progenitor masses around 120 solar masses or even above, making them the direct descendants of the most massive stars in the galaxy. Because of the proximity to the Eddington limit our models are very sensitive to the LM ratio, thus allowing for a direct estimate of the present masses of these objects. |
The Axis of Evil revisited ; In light of the threeyear data release from WMAP we reexamine the evidence for the Axis of Evil'' AOE. We discover that previous statistics are not robust with respect to the datasets available and different treatments of the galactic plane. We identify the cause of the instability and implement an alternative model selection'' approach. A comparison to Gaussian isotropic simulations find the features significant at the 9498 level, depending on the particular AOE model. The Bayesian evidence finds lower significance, ranging from substantial'' at Deltaln Esim 1.4, to no evidence for the most general AOE model. |
Stellar Evolutionary Models challenges from observations of stellar systems ; We briefly review some constraints Owing to the limited number of pages of present review, only a subsample of the topics discussed during the talk are briefly summarized. For the interested readers we are pleased to send them upon request the complete presentation file. for stellar models in various mass regimes and evolutionary stages as provided by observational data from spectroscopy to multiwavelenghts photometry. The accuracy of present generation of stellar models can be significantly improved only through an extensive comparison between theory and observations. |
A general algebraic model for molecular vibrational spectroscopy ; We introduce the Anharmonic Oscillator Symmetry Model to describe vibrational excitations in molecular systems exhibiting high degree of symmetry. A systematic procedure is proposed to establish the relation between the algebraic and configuration space formulations, leading to new interactions in the algebraic model. This approach incorporates the full power of group theoretical techniques and provides reliable spectroscopic predictions. We illustrate the method for the case of cal D3htriatomic molecules. |
Concurrent Lexicalized Dependency Parsing The ParseTalk Model ; A grammar model for concurrent, objectoriented natural language parsing is introduced. Complete lexical distribution of grammatical knowledge is achieved building upon the headoriented notions of valency and dependency, while inheritance mechanisms are used to capture lexical generalizations. The underlying concurrent computation model relies upon the actor paradigm. We consider message passing protocols for establishing dependency relations and ambiguity handling. |
Tracking Initiative in Collaborative Dialogue Interactions ; In this paper, we argue for the need to distinguish between task and dialogue initiatives, and present a model for tracking shifts in both types of initiatives in dialogue interactions. Our model predicts the initiative holders in the next dialogue turn based on the current initiative holders and the effect that observed cues have on changing them. Our evaluation across various corpora shows that the use of cues consistently improves the accuracy in the system's prediction of task and dialogue initiative holders by 24 and 813 percentage points, respectively, thus illustrating the generality of our model. |
Critical exponents of the degenerate Hubbard model ; We study the critical behaviour of the SUN generalization of the onedimensional Hubbard model with arbitrary degeneracy N. Using the integrability of this model by Bethe Ansatz we are able to compute the spectrum of the lowlying excitations in a large but finite box for arbitrary values of the electron density and of the Coulomb interaction. This information is used to determine the asymptotic behaviour of correlation functions at zero temperature in the presence of external fields lifting the degeneracy. The critical exponents depend on the system parameters through a Ntimes N dressed charge matrix implying the relevance of the interaction of charge and spindensity waves. |
SpinCharge separation in a model of two coupled chains ; A model of interacting electrons living on two chains coupled by a transverse hopping tperp, is solved exactly by bosonization technique. It is shown that tperp does modify the shape of the Fermi surface also in presence of interaction, although charge and spin excitations keep different velocities urho, usigma. Two different regimes occur at short distances, xll xi urho usigma4tperp, the two chain model is not sensitive to tperp, while for larger separation xgg xi interchain hopping is relevant and generates further singularities in the electron Green function besides those due to spincharge decoupling. 2 figures not included. Figure requests FABRIZIOITSSISSA |
Even and oddfrequency pairing correlations in 1D tJh model a comparative study ; An equal time version of oddfrequency pairing for a generalized tJ model is introduced. It is shown that the composite operators describing binding of Cooper pairs with magnetization fluctuations naturally appear in this approach. The pairing correlations in both BCS and oddfrequency channels are investigated exactly in 1D systems with up to 16 sites. Our results indicate that at some range of parameters oddfrequency correlations become comparable, however smaller than BCS pairing correlations. It is speculated that the spin and density fluctuations in the frustrated model lead to the enhancement of the odd gap susceptibilities. 4 postscript figure files are attached at the bottom of the tex file. |
Vertex Models and Quantum Spin Systems a nonlocal approach ; Within a general cluster framework, we discuss the loopalgorithm, a new type of cluster algorithm that reduces critical slowing down in vertex models and in quantum spin systems. We cover the example of the 6vertex model in detail. For the Fmodel, we present numerical results that demonstrate the effectiveness of the loop algorithm. We discuss how to modify the original algorithm for some more complicated situations, especially for quantum spin systems in one and two dimensions. |
Integrable versus NonIntegrable Spin Chain Impurity Models ; Recent renormalization group studies of impurities in spin12 chains appear to be inconsistent with Bethe ansatz results for a special integrable model. We study this system in more detail around the integrable point in parameter space and argue that this integrable impurity model corresponds to a nongeneric multicritical point. Using previous results on impurities in halfinteger spin chains, a consistent renormalization group flow and phase diagram is proposed. |
Renormalization and Hyperscaling for SelfAvoiding Manifold Models ; The renormalizability of the selfavoiding manifold SAM Edwards model is established. We use a new short distance multilocal operator product expansion MOPE, which extends methods of local field theories to a large class of models with nonlocal singular interactions. This validates the direct renormalization method introduced before, as well as scaling laws. A new general hyperscaling relation for the configuration exponent gamma is derived. Manifolds at the Thetapoint, and long range Coulomb interactions are briefly discussed. |
Formation of SpaceTime Structure in a ForestFire Model ; We present a general stochastic forestfire model which shows a variety of different structures depending on the parameter values. The model contains three possible states per site tree, burning tree, empty site and three parameters tree growth probability p, lightning probability f, and immunity g. We review analytic and computer simulation results for a quasideterministic state with spiralshaped fire fronts, for a percolationlike phase transition and a selforganized critical state. Possible applications to excitable systems are discussed. |
Effective lowering of the dimensionality in strongly correlated two dimensional electron gas ; We study a system of fermions interacting with a gauge field which can be used to describe either spin liquid or nu12 Quantum Hall state. We propose a generalized model with a dimensionless parameter N. We evaluate the properties of the model in both limits N gg 1 and N ll 1 and deduce the properties of the model in the most physically intersting case of N1,2. At N ll 1 the motion of the fermions becomes one dimensional. This allows us to obtain the fermion Green function and response functions applying bozonization method in this limit. |
Exactly solvable random matrix models with additional twobody interactions ; It has been argued that despite remarkable success, existing random matrix theories are not adequate to describe disordered conductors in the metallic regime, due to the presence of certain twobody interactions in the effective Hamiltonian for the eigenvalues, in addition to the standard logarithmic interaction that arises entirely from symmetry considerations. We present a new method that allows exact solution of random matrix models with such additional twobody interactions. This should broaden the scope of random matrix models in general. |
Superconductivity in the cuprates ; We evaluate numerically several superconducting correlation functions in a generalized tJ model derived for holedoped CuO2 planes. The model includes a threesite term t'' similar to that obtained in the large U limit of the Hubbard model but of opposite sign for realistic OO hopping. For realistic parameters we obtain strong evidence of superconductivity of predominantly dx2y2 character. The ground state has a large overlap with a very simple resonatingvalencebond wave function with offdiagonal longrange order. This function reproduces the main features of the magnetic and superconducting correlation functions. |
Phase Transitions of Frustrated XY spins in Two Dimensions ; The row model is used to study the commensurateincommensurate CIC an isotropic FFTXY transitions of the frustrated 2D XY model on the triangular lattice. New relevant variables clarify the physics of these transitions phase and chiral variables are coupled so that spin waves generate long range polar interactions. The resulting dielectric constan diverges at the transition. A single transition occurs for the FFTXY model; in the CIC regime the Lifshitz point is at T0 and the C phase i a SmecticA like phase which disorders via a 2D nematicsmecticA transition. |
Fermi and NonFermi behavior in the anisotropic Multichannel Kondo Model Bethe Ansatz solution ; We solve the Multichannel Kondo model with channel anisotropy using the Bethe Ansatz method. The model generates energy scales, characterizing the neighborhoods of the various infrared fixed points, in correspondence to the structure of the symmetry breaking in the channel sector. The nature of these fixed points also depends on the magnitude of the impurity spin S. We present a detailed discussion for the two channel case. |
Weak Coupling Phase Diagram of the Two Chain Hubbard Model ; We present a general method for determining the phase diagram of systems of a finite number of one dimensional Hubbardlike systems coupled by singleparticle hopping with weak interactions. The technique is illustrated by detailed calculations for the twochain Hubbard model, providing the first controlled results for arbitrary doping and interchain hopping. Of nine possible states which could occur in such a spin12 ladder, we find seven at weak coupling. We discuss the conditions under which the model can be regarded as a onedimensional analog of a superconductor. |
Scaling Law for a Magnetic Impurity Model with TwoBody Hybridization ; We consider a magnetic impurity coupled to the hybridizing and screening channels of a conduction band. The model is solved in the framework of poor man's scaling and Cardy's generalized theories. We point out that it is important to include a twobody hybridization if the scaling theory is to be valid for the band width larger than U. We map out the boundary of the FerminonFermi liquid phase transition as a function of the model parameters. |
Glassiness in a model without energy barriers ; We propose a microscopic model without energy barriers in order to explain some generic features observed in structural glasses. The statics can be exactly solved while the dynamics has been clarified using Monte Carlo calculations. Although the model has no thermodynamic transition it captures some of the essential features of real glasses, i.e., extremely slow relaxation, time dependent hysteresis effects, anomalous increase of the relaxation time and aging. This suggests that the effect of entropy barriers can be an important ingredient to account for the behavior observed in real glasses. |
SpinCharge Separation at Finite Temperature in the Supersymmetric tJ Model with LongRange Interactions ; Thermodynamics is derived rigorously for the 1D supersymmetric it tJ model and its SUK,1 generalization with inversesquare exchange. The system at low temperature is described in terms of spinons, antispinons, holons and antiholons obeying fractional statistics. They are all free and make the spin susceptibility independent of electron density, and the charge susceptibility independent of magnetization. Thermal spin excitations responsible for the entropy of the SUK,1 model are ascribed to free parafermions of order K1. |
A note on inversesquare exchange models ; The su11 symmetric version of the HaldaneShastry spin chain is diagonalized by means of a linear transformation. The same transformation applied to the original su2 model yields simple expressions for the Hamiltonian and the generators of the Yangian symmetry of the model in terms of spin wave operators. |
On Damage Spreading Transitions ; We study the damage spreading transition in a generic onedimensional stochastic cellular automata with two inputs DomanyKinzel model Using an original formalism for the description of the microscopic dynamics of the model, we are able to show analitically that the evolution of the damage between two systems driven by the same noise has the same structure of a directed percolation problem. By means of a mean field approximation, we map the density phase transition into the damage phase transition, obtaining a reliable phase diagram. We extend this analysis to all symmetric cellular automata with two inputs, including the Ising model with heathbath dynamics. |
Global Bethe lattice consideration of the spin1 Ising model ; The spin1 Ising model with bilinear and biquadratic exchange interactions and singleion crystal field is solved on the Bethe lattice using exact recursion equations. The general procedure of critical properties investigation is discussed and full set of phase diagrams are constructed for both positive and negative biquadratic couplings. In latter case we observe all remarkable features of the model, uncluding doublyreentrant behavior and ferrimagnetic phase. A comparison with the results of other approximation schemes is done. |
On Matrix Product Ground States for ReactionDiffusion Models ; We discuss a new mechanism leading to a matrix product form for the stationary state of onedimensional stochastic models. The corresponding algebra is quadratic and involves four different matrices. For the example of a coagulationdecoagulation model explicit fourdimensional representations are given and exact expressions for various physical quantities are recovered. We also find the general structure of npoint correlation functions at the phase transition. |
Surface Critical Phenomena in InteractionRoundaFace Models ; A general scheme has been proposed to study the critical behaviour of integrable interactionroundaface models with fixed boundary conditions. It has been shown that the boundary crossing symmetry plays an important role in determining the surface free energy. The surface specific heat exponent can thus be obtained without explicitly solving the reflection equations for the boundary face weights. For the restricted SOS Lstate models of Andrews, Baxter and Forrester the surface specific heat exponent is found to be alphas2L14. |
Charged particles in random magnetic fields and the critical behavior in the fractional quantum Hall effect ; As a model for the transitions between plateaus in the fractional Quantum Hall effect we study the critical behavior of noninteracting charged particles in a static random magnetic field with finite mean value. We argue that this model belongs to the same universality class as the integer Quantum Hall effect. The universality is proved for the limiting cases of the lowest Landau level, and slowly fluctuating magnetic fields in arbitrary Landau levels. The conjecture that the universality holds in general is based on the study of the statistical properties of the corresponding random matrix model. |
Control of the finite size corrections in exact diagonalization studies ; We study the possibility of controlling the finite size corrections in exact diagonalization studies quantitatively. We consider the one and two dimensional Hubbard model. We show that the finitesize corrections can be be reduced systematically by a grandcanonical integration over boundary conditions. We find, in general, an improvement of one order of magnitude with respect to studies with periodic boundary conditions only. We present results for groundstate properties of the 2D Hubbard model and an evaluation of the specific heat for the 1D and 2D Hubbard model. |
Orientational Glass Transition ; The static behavior of orientational glasses is discussed in terms of a replica theory based on the infinite range random bondrandom field model. A general version of the model applicable to dipolar and quadrupolar glasses is presented using a symmetryadapted representation for the order parameter fields. Numerical results for the 100 quadrupolar glass are obtained and compared with the Ising and 111 orientational glass models. |
Exact Solution of an One Dimensional Deterministic Sandpile Model ; Using the transfer matrix method, we give the exact solution of a deterministic sandpile model for arbitrary N, where N is the size of a single toppling. The one and twopoint functions are given in term of the eigenvalues of an N times N transfer matrix. All the npoint functions can be found in the same way. Application of this method to a more general class of models is discussed. We also present a quantitative description of the limit cycle attractor as a multifractal. |
Exact Solution of an Irreversible OneDimensional Model with Fully Biased Spin Exchanges ; We introduce a model with conserved dynamics, where nearest neighbor pairs of spins uparrow downarrow downarrow uparrow can exchange to assume the configuration downarrow uparrow uparrow downarrow, with rate beta alpha, through energy decreasing moves only. We report exact solution for the case when one of the rates, alpha or beta, is zero. The irreversibility of such dynamics results in strong dependence on the initial conditions. Domain wall arguments suggest that for more general models with steady states the dynamical critical exponent for the anisotropic spin exchange is different from the isotropic value. |
Closure of the Monte Carlo dynamical equations in the spherical SherringtonKirkpatrick model ; We study the analytical solution of the Monte Carlo dynamics in the spherical SherringtonKirkpatrick model using the technique of the generating function. Explicit solutions for onetime observables like the energy and twotime observables like the correlation and response function are obtained. We show that the crucial quantity which governs the dynamics is the acceptance rate. At zero temperature, an adiabatic approximation reveals that the relaxational behavior of the model corresponds to that of a single harmonic oscillator with an effective renormalized mass. |
On the flux phase conjecture at halffilling an improved proof ; We present a simplification of Lieb's proof of the flux phase conjecture for interacting fermion systems such as the Hubbard model , at half filling on a general class of graphs. The main ingredient is a procedure which transforms a class of fermionic Hamiltonians into reflection positive form. The method can also be applied to other problems, which we briefly illustrate with two examples concerning the tV model and an extended FalicovKimball model. |
Integer Quantum Hall Effect in DoubleLayer Systems ; We consider the localization of independent electron orbitals in doublelayer twodimensional electron systems in the strong magnetic field limit. Our study is based on numerical Thouless number calculations for realistic microscopic models and on transfer matrix calculations for phenomenological network models. The microscopic calculations indicate a crossover regime for weak interlayer tunneling in which the correlation length exponent appears to increase. Comparison of network model calculations with microscopic calculations casts doubt on their generic applicability. |
Supersymmetric matrix models and branched polymers ; We solve a supersymmetric matrix model with a general potential. While matrix models usually describe surfaces, supersymmetry enforces a cancellation of bosonic and fermionic loops and only diagrams corresponding to socalled branched polymers survive. The eigenvalue distribution of the random matrices near the critical point is of a new kind. |
Surface Magnetisation and Surface Correlations in Aperiodic Ising Models ; We consider the surface critical behaviour of diagonally layered Ising models on the square lattice where the interlayer couplings follow some aperiodic sequence. The surface magnetisation is analytically evaluated from a simple formula derived by the diagonal transfer matrix method, while the surface spinspin correlations are obtained numerically by a recursion method, based on the startriangle transformation. The surface critical behaviour of different aperiodic Ising models are found in accordance with the corresponding relevanceirrelevance criterion. For marginal sequences the critical exponents are continuously varying with the strength of aperiodicity and generally the systems follow anisotropic scaling at the critical point. |
Cooperativity of Protein Folding and the RandomField Ising Model ; The relation between cooperativity of protein folding and the RandomField Ising Model RFIM is established. Generalization of the ImryMa argument predicts cooperative folding transition for small heterogeneity of the interactions stabilizing the native structure. Monte Carlo simulation of a lattice model shows that starting from some finite heterogeneity folding transition is not cooperative and involves formation of domains. |
Surface Free Energies, Interfacial Tensions and Correlation Lengths of the ABF Models ; The surface free energies, interfacial tensions and correlation lengths of the AndrewsBaxterForrester models in regimes III and IV are calculated with fixed boundary conditions. The interfacial tensions are calculated between arbitrary phases and are shown to be additive. The associated critical exponents are given by 2alphasmunu with nuL14 in regime III and 42alphasmunu with nuL12 in regime IV. Our results are obtained using general commuting transfer matrix and inversion relation methods that may be applied to other solvable lattice models. |
Hubbard Models with Superconducting Quantum Symmetry ; We discuss classical and quantum symmetries of extended Hubbard models. The quantum symmetries are shown to be related to the known superconducting SU2 symmetry of the original Hubbard model at half filling via generalized LangFirsov transformations and Drinfel'd twists. Based on talk at the 21st ICGTM in Goslar, July 1996 |
Duality Relations for Potts Correlation Functions ; Duality relations are obtained for correlation functions of the qstate Potts model on any planar lattice or graph using a simple graphical analysis. For the twopoint correlation we show that the correlation length is precisely the surface tension of the dual model, generalizing a result known to hold for the Ising model. For the threepoint correlation an explicit expression is obtained relating the correlation function to ratios of dual partition functions under fixed boundary conditions. |
A New Family of Integrable Extended Multiband Hubbard Hamiltonians ; We consider exactly solvable 1d multiband fermionic Hamiltonians, which have affine quantum group symmetry for all values of the deformation. The simplest Hamiltonian is a multiband tJ model with vanishing spinspin interaction, which is the affinization of an underlying XXZ model. We also find a multiband generalization of standard tJ model Hamiltonian. |
Anomalous Drude Model ; A generalization of the Drude model is studied. On the one hand, the free motion of the particles is allowed to be sub or superdiffusive; on the other hand, the distribution of the time delay between collisions is allowed to have a long tail and even a nonvanishing first moment. The collision averaged motion is either regular diffusive or L'evyflight like. The anomalous diffusion coefficients show complex scaling laws. The conductivity can be calculated in the diffusive regime. The model is of interest for the phenomenological study of electronic transport in quasicrystals. |
Wetting phenomena in bcc binary alloys ; We study the influence of the surface orientation on the wetting behavior of bcc binary alloys, using a semiinfinite lattice model equivalent to a nearestneighbor Ising antiferromagnet in an external field. The salient feature of the model is the generation of an effective'' ordering surface field for symmetrybreaking surface orientations like 100. Such a field couples to the local order parameter at the surface and leads to the occurrence of wetting phenomena below the critical temperature Tc. Utilizing a GinzburgLandau continuum model which has been derived earlier from the lattice meanfield theory, the wetting phase diagram is calculated. |
Exact Results for a Kondo Problem in One Dimensional tJ Model ; We propose an integrable Kondo problem in a onedimensional 1D tJ model. With the open boundary condition of the wave functions at the impurity sites, the model can be exactly solved via Bethe ansatz for a class of JR,L Kondo coupling constants and VL,R impurity potentials parametrized by a single parameter c. The integrable value of JL,R runs from negative infinity to positive infinity, which allows us to study both the ferromagnetic Kondo problem and antiferromagnetic Kondo problem in a strongly correlated electron system. Generally, there is a residual entropy for the ground state, which indicates a typical nonFermi liquid behavior. |
An Ultimate Frustration in ClassicalLattice Gas Models ; We constructed an uncountable family of classical latticegas models with unique groundstate measures which are not uniquely ergodic measures of any tiling system, or more generally, of any system of finite type. Therefore, we have shown that the family of structures which are unique ground states of some translationinvariant, finiterange interactions is larger than the family of tilings which form single isomorphism classes. Such groundstate measures cannot be groundstate measures of any translationinvariant, finiterange, nonfrustrated potential. Our groundstate configurations are twodimensional analogs of onedimensional, most homogeneous groundstate configurations of infiniterange, convex, repulsive interactions in models with devil's staircases. |
LenzIsingOnsager problem in an external field as a soluble problem of many fermions ; In this paper a new approach to solving the 2D and 3D Ising models in external magnetic field Hneq0 is developed. The general formalism for the approach to the problem is presented on the example of the 2D Ising model in the external magnetic field. The paper presents a new method obtaining the Onsager solution and computations of asymptotic forms of lowtemperature free energy for the 2D Ising model in the external magnetic field H. The free energy in the limiting case of the magnetic field tending to zero Hto 0, N,Mtoinfty at arbitrary temperature is also considered Tneq 0. |
Mixed spin ladders with exotic ground states ; We study the mixed spin isotropic ladder system having S1 spins on one leg and S12 spins on the other, with generaltype exchange interactions between spins on neighboring rungs. A set of model Hamiltonians with exact ground states in the form of a certain matrix product wave function is obtained. We show that sufficiently strong frustration can lead to exotic singlet ground states with infinite exponential degeneracy. We also list a couple of rather simple models with nontrivial ground states, including a model with only bilinear exchange. |
A 1d Traffic Model with Threshold Parameters ; The basic properties of traffic flow are analyzed using a simple deterministic one dimensional car following model with continuous variables based on a model introduced by Nagel and Herrmann Physica A 199 254269 1993 including a few modifications. As a first case we investigate the creation and propagation of jams in a platoon generated by a slow leading vehicle. In a second case we look at a system with the size L, periodic boundary conditions and identical vehicles. A strong dependence on the initial configuration of the fundamental diagram's shape can be found. |
Models of impurities in valence bond spin chains and ladders ; We present the class of models of a nonmagnetic impurity in S12 generalized ladder with an AKLTtype valence bond ground state, and of a S12 impurity in the S1 AKLT chain. The ground state in presence of impurity can be found exactly. Recently studied phenomenon of local enhancement of antiferromagnetic correlations around the impurity is absent for this family of models. |
A Density Matrix Algorithm for 3D Classical Models ; We generalize the corner transfer matrix renormalization group, which consists of White's density matrix algorithm and Baxter's method of the corner transfer matrix, to three dimensional 3D classical models. The renormalization group transformation is obtained through the diagonalization of density matrices for a cubic cluster. A trial application for 3D Ising model with m2 is shown as the simplest case. |
Metastable States in Cellular Automata for Traffic Flow ; Measurements on real traffic have revealed the existence of metastable states with very high flow. Such states have not been observed in the NagelSchreckenberg NaSch model which is the basic cellular automaton for the description of traffic. Here we propose a simple generalization of the NaSch model by introducing a velocitydependent randomization. We investigate a special case which belongs to the socalled slowtostart rules. It is shown that this model exhibits metastable states, thus sheding some light on the prerequisites for the occurance of hysteresis effects in the flowdensity relation. |
CurrentInduced Step Bending Instability on Vicinal Surfaces ; We model an apparent instability seen in recent experiments on current induced step bunching on Si111 surfaces using a generalized 2D BCF model, where adatoms have a diffusion bias parallel to the step edges and there is an attachment barrier at the step edge. We find a new linear instability with novel step patterns. Monte Carlo simulations on a solidonsolid model are used to study the instability beyond the linear regime. |
Connection between CalogeroMarchioroWolfes type fewbody models and free oscillators ; We establish the exact correspondence of the CalogeroMarchioroWolfes model and several of its generalizations with free oscillators. This connection yields the eigenstates and leads to a proof of the quantum integrability. The usefulness of our method for finding new solvable models is then demonstrated by an example. |
FiniteSize Bosonization of 2Channel Kondo Model a Bridge between Numerical Renormalization Group and Conformal Field Theory ; We generalize Emery and Kivelson's EK bosonizationrefermionization treatment of the 2channel Kondo model to finite system size and on the EKline analytically construct its exact eigenstates and finitesize spectrum. The latter crosses over to conformal field theory's CFT universal nonFermiliquid spectrum and yields the mostrelevant operators' dimensions, and further to a Fermiliquid spectrum in a finite magnetic field. Our approach elucidates the relation between bosonization, scaling techniques, the numerical renormalization group NRG and CFT. All CFT's Green's functions are recovered with remarkable ease from the model's scattering states. |
Internal Avalanches in a Granular Medium ; Avalanches of grain displacements can be generated by creating local voids within the interior of a granular material at rest in a bin. Modeling such a twodimensional granular system by a collection of monodisperse discs, the system on repeated perturbations, shows all signatures of SelfOrganized Criticality. During the propagation of avalanches the competition among grains creates arches and in the critical state a distribution of arches of different sizes is obtained. Using a cellular automata model we demonstrate that the existence of arches determines the universal behaviour of the model system. |
Hidden Kondo Effect in a Correlated Electron Chain ; We develop a general Bethe Ansatz formalism for diagonalizing an integrable model of a magnetic impurity of arbitrary spin coupled ferro or antiferromagnetically to a chain of interacting electrons. The method is applied to an open chain, with the exact solution revealing a hidden'' Kondo effect driven by forward electron scattering off the impurity. We argue that the socalled operator reflection matrices'' proposed in recent Bethe Ansatz studies of related models emulate only forward electronimpurity scattering which may explain the absence of complete Kondo screening for certain values of the impurityelectron coupling in these models. |